1. Field of the Invention
The present invention relates to an improved Q factor-built-in RFC (Radio frequency coil) for magnetic resonance imaging (MRI) apparatus including a dielectric component.
2. Description of the Related Art
As an RF coil for MRI apparatus, use has generally been made of a coil called "STR (Slot Tube Resonator)" as shown in FIG. 6.
In FIG. 6, an RF coil 1 comprises a bobbin 2 composed of a cylindrical dielectric unit, conductor bands 3 oppositely arranged, as coil conductors, at four places on the outer periphery of the bobbin 2 in the longitudinal direction of the bobbin 2, and a pair of guard rings 4 provided one at each inner end portion of the bobbin 2. The conductor band 3 and guard ring 4, being greater in their width, lead to a smaller inductance as a merit. The conductor band 3 and guard ring 4 can be so designed as to be employed under a high magnetic field of over 0.5 T (tesla), that is, under a resonant frequency of 21.3 MHz in a hydrogen atom.
In medical diagnosis, the RF coil 1 enables a radiofrequency wave to be transmitted and received to and from a living subject, that is, a subject to be examined, located in the cylindrical unit and hence acquires an MR signal. The MR (Magnetic Resonance) signal thus acquired is supplied to an image forming unit in an MRI apparatus where it is reconstituted through a two-dimensional Fourier transformation. The reconstituted image is supplied to an output unit, that is, an image display unit, where an MRI is displayed.
Upon transmission and reception to and from the subject, the RF coil 1 is so operated as indicated by an equivalent circuit regarding the RF coil in FIG. 7. Stated in more detail, a current flows in the circuit in a direction indicated by the solid and dotted lines in FIG. 7, enabling MR signals to be detected along two channels CH.sub.1 and CH.sub.2. This circuit can gain an S/N (signal to noise) ratio .sqroot.2 times as great as that in the case where such detection is made along one channel.
The bobbin 2, per se, at those portions 5 between the conductor band 3 and the guard ring 4 provide electrostatic capacitance C.sub.1. Let it be assumed that, as will be seen from a concept diagram in FIG. 8, an area at the bobbin portion 5 between the coil conductor 3 and the guard ring 4 is represented by A.sub.1, a distance (thickness) between the coil conductor 3 and the guard ring 4 by N and the dielectric constant at the aforementioned portion of the bobbin by .epsilon.. Then the following relation is established: EQU Cl=.epsilon..multidot.(A.sub.1 /L)
Upon the transmission and reception of the RF waves to and from the coil 1, electric lines of force are concentrated at the bobbin portion 5, creating a dielectric loss. According to experiments conducted by the inventor, it has been found that a Q-factor, in a nonload state, of the coil 1 is as low as 80 (measured data).
Let it be assumed that the dielectric loss can be reduced by setting, smaller, the electrostatic capacitance C.sub.1 at the bobbin portion 5. Then the Q-factor is improved or increased, resulting in an improvement in the S/N ratio of the MR image.
The reduction of the electrostatic capacitance C.sub.1 at the bobbin portion 5 can be achieved by setting, greater, the distance L between the coil conductors 3 and 4. However, the coil 1 becomes bulkier by doing so. Further, if the area A.sub.1 contacting with the bobbin at the bobbin portion 5 defined above is made smaller, the conductive loss is increased, failing to obtain an improved Q-factor. The bobbin 2, being formed of a dielectric of smaller dielectric constant, such as Teflon (registered trademark), results in added costs in apparatus, due to a restriction imparted to a material of which the bobbin 2 is formed.
The aforementioned conventional coil cannot obtain an improved Q-factor because a greater influence is exerted by the electrostatic capacitance of the dielectric bobbin portion between the coil conductors 3 and 4.